A subsea well assembly includes a wellhead housing that is secured to large diameter conductor pipe extending to a first depth in the well. After drilling to a second depth, a string of casing is lowered into the well and suspended in the wellhead housing by a casing hanger. A packoff seals between an outer diameter portion of the casing hanger and the bore of the wellhead housing. Some wells have two or more strings of casing, each supported by a casing hanger in the wellhead housing.
In one type of completion, a string of production tubing is lowered into the last string of casing. A tubing hanger lands and seals in the upper casing hanger. The well is produced through the tubing. Prior to running the tubing, the operator will test the upper casing hanger packoff. On rare occasions, the packoff may be unable to pass the pressure test, possibly due to damage on the interior wall of the wellhead housing. If so, one remedy is to install an emergency or bridging hanger in the wellhead housing. The bridging hanger does not support a string of casing, but has an interior profile that is normally the same as the profile in the upper casing hanger. The operator lands and seals the lower portion of the bridging hanger to the casing hanger. The operator installs a packoff between the upper exterior portion of the bridging hanger and the wellhead housing above the casing hanger. The operator then runs the tubing and lands and seals the tubing hanger in the bridging hanger.
In the prior art, the inner seal between the bridging hanger and the casing hanger is normally elastomeric. As the bridging hanger enters the casing hanger, the elastomeric seal deforms to cause the sealing engagement. Metal-to-metal outer seals or packoffs have been used for years because they can withstand higher pressures than elastomeric seals and also do not deteriorate under harsh environments as readily. Metal-to-metal tubing hanger seals are also employed in many wells. A metal-to-metal seal, however, typically requires much more force to set than simply the weight of the running string. Various running tools have been developed to apply the high forces needed. Developing a running tool to set a metal-to-metal inner seal would require an additional trip down the riser with another running tool to set the metal-to-metal outer seal. In offshore wells, particularly in deep water, it is very expensive to run an additional trip.